PM2 will keep applications running, as long as it is running itself. But if PM2 itself is terminated (for example, as a result of a reboot), then we must also configure PM2 to automatically restart. Very conveniently, PM2 provides a startup command, which outputs a script for you to run on your terminal:

hobnob@hobnob:$ npx pm2 startup
[PM2] Init System found: systemd
[PM2] To setup the Startup Script, copy/paste the following command:
sudo env PATH=$PATH:/home/hobnob/.nvm/versions/node/v8.11.4/bin /home/hobnob/projects/hobnob/node_modules/pm2/bin/pm2 startup systemd -u hobnob --hp /home/hobnob

Run the script to ensure PM2 starts on boot. Now, when you log out of your terminal session, or when the application crashes unexpectedly, or even the whole machine restarts, you can be confident that your application will automatically restart as soon as possible.